1. Field of the Invention
The present invention relates to a bypass damper for use with a series-type ventilation fan. More specifically, the invention relates a damper designed to prevent the backspin of the fan.
2. Description of the Prior Art
A fan terminal is a popular product in the commercial heating, ventilation and air conditioning (HVAC) market. These fans are located in the ceiling, as part of the distribution ductwork near the space being ventilated. The function of the fan terminal is to draw in warm ceiling return air, when needed. Two types of fan terminals serve this purpose: the parallel-type and the series-type. In the parallel-type fan terminal, the fan is not located in the primary ventilation air flow path. The fan remains off until heat is needed, then it is energized and draws warm air from above the ceiling into its intake and discharges it into the ductwork downstream of a primary ventilation damper. In this case, both the primary damper and the fan are controlled by a thermostat in the space.
A series-type fan, on the other hand, is physically located in the primary ventilation air flow path. In this type of unit, the fan runs continuously. The primary damper discharges its air into a plenum surrounding the series fan, which is also open to the ceiling area. The suction effect of the fan prevents ventilation air from spilling out into the ceiling area. Typically, the capacity of the fan is variable and is adjusted to match the maximum cooling requirements of the space. Under the maximum cooling conditions, all of the cooling ventilation air is drawn into the fan intake and is discharged into ductwork downstream of the fan. This ductwork then feeds cooling air into the space. As the cooling requirements decrease, and the primary damper closes, the fan draws warmer ceiling air into the plenum, mixed with the reduced cooling air flow, and discharges this mix air into the downstream ductwork. The air flow to the space remains relatively constant, however the mix of cooled air to warmer ceiling air is changed.
A problem exists in a series-type fan terminal when the fan is shut off and restarted while the primary air flow exists. Series-type fan terminals typically use a forward curved blower and direct drive, permanent split capacitor (PSC) motor for low cost and high efficiency. If air is pushed through a forward curve blower that is de-energized, the blower will spin backward. If enough backward speed is generated, the PSC motor will run backward when energized. If this happens, the fan becomes loud, the blower wheel may loosen, and the motor may burn out. A series-type fan is susceptible to such problems when there is sufficient cooling air flow when the motor is de-energized (if the terminal was in its full heating mode, i.e. the primary damper closed, the fan, would restart properly), and the fan power is interrupted for a minimum of 20 to 30 seconds.